27 



Although so connected lithologically and stratigraphically as 

 to be clearly members of one great series, yet these various 

 groups are, on the whole, well separated, occurring mainly in 

 large masses. The stratigraphic distinctness would be much 

 more striking but for the wide-spread extravasation which some 

 of the divisions have experienced. Their general separateness 

 implies that they are, for the most part, of different ages, 

 — are chronologically distinct ; and may each be regarded 

 as a sort of sub-formation. Whence it follows that, in all 

 their more special relations, they admit of separate description; 

 and that is the plan adopted here. The different groups will be 

 taken up in regular order, beginning with the oldest. The true 

 sequence, excluding some of the limestone, is expressed in the 

 foregoing classification. 



GKANITE. 



The typical granite of this region, as shown at the quarries 

 in Quincy and other places, is a coarsely crystalline aggregate 

 of orthoclase, quartz, and hornblende. Orthoclase is the pre- 

 dominant mineral, and in its abundance constitutes the leading 

 character of the rock ; this is pre-eminently a feldspathic gran- 

 ite. The hornblende is usually small in amount, and the rock 

 frequently passes, through the disappearance of hornblende, into 

 binary granite. All the Huronian granites of this region, and 

 especially this typical variety, are remarkably firm and coherent, 

 being strongly contrasted, in this respect, with the most of the 

 granite of the Montalban and newer formations. InTopsfield, 

 in and near the village, the granite is locally rather loose and 

 friable ; but this is the only instance of the kind within my 

 knowledge which is not clearly the result of atmospheric action. 

 And it may be said that these granites, as a rule, resist chemi- 

 cal quite as well as mechanical forces. The colors of the gran- 

 ite are mainly due to the feldspar, the hornblende seldom being 

 sufficiently abundant to sensibly darken the tint of the aggre- 

 gate. The feldspar is usually grayish or bluish, though differ- 



